The present invention relates to a process for preparing cordierite. More particularly, the invention relates to a process for preparing cordierite used for materials wherein chemical stability, heat resistance, low coefficient of thermal expansion, thermal shock resistance, etc., are required, such as exhaust gas cleaning catalysts for automobiles, carriers for combustion catalysts for fan heaters and heat exchangers for gas turbine engines, and materials wherein insulation properties and low dielectric constant are required, such as IC substrate materials for high-speed devices.
Cordierite ceramics having a high thermal impact resistance have hitherto been used for honeycomb carriers widely used as exhaust gas cleaning catalysts for automobiles and carriers for combustion catalysts of domestic fan heaters. There is an ever-increasing demand for the cordierite due to an increase in the production of automobiles and various combustion equipment. Further, the cordierite has drawn attention also as IC substrate materials for high speed devices by virtue of its low dielectric constant and high insulation properties.
However, in prior art processes, such as a process wherein a starting powder mixture is sintered to prepare a sinter and a process wherein a starting mixture is melted and crystallized from glass, the temperature of forming .alpha.-cordierite is high and the sintering temperature is very close to the incongruent melting temperature (1445.degree. C.), so that it is difficult to prepare a cordierite sinter in a pure single phase. For this reason, in general, a sintering aid, such as titanium oxide or an oxide of an alkali metal or an alkaline earth metal has been added.
However, in this method, it is difficult to control the degree of sintering, which brings about an increase in both the coefficient of thermal expansion and dielectric constant of the resultant cordierite.
In recent years, a partial hydrolysis process for an ester of silicic acid has been reported as a process by which cordierite can be formed at a temperature below that used in the conventional process (see Journal of Ceramic Society of Japan, 1987, vol. 95, pp. 163-169; ibid., pp. 169-175).
The above-described partial hydrolysis process is one wherein among alkoxides of silicon, aluminum and magnesium used as starting materials, ethyl silicate which exhibits a particularly low hydrolysis rate is preliminarily hydrolyzed with a small amount of dilute hydrochloric acid. According to this process, it is possible to prepare .alpha.-cordierite in a single phase through sintering at 1200.degree. C. for 2 hr.
In this process, however, a very long reaction time, i.e., 100 hr at 70.degree. C., is necessary for partial hydrolysis of tetraethoxysilane. Further, if the partial hydrolysis time is shortened to one half, no pure .alpha.-cordierite can be prepared even after sintering at 1400.degree. C. for 2 hr and instead a mixture of many compounds is formed. Therefore, in order to prepare a pure product, it is necessary to carefully control the reaction.
In the above-described hydrolysis process, it may be anticipated that a part or whole of the alkoxy groups of the ester might be hydrolyzed to give a silanol type ester [see the following formula (1)]. EQU Si(OR).sub.4 +.sub.n H.sub.2 O.fwdarw.Si(OR).sub.4-n (OH).sub.n +.sub.n ROH(1)
In practice, however, it is known that the silanol type chemical species is converted into a high-molecular weight chemical species through polycondensation [see the following formula (2); J. C. Pouxviel et al., Journal of Non-Crystalline Solids, 89, 345-360 (1987)]. ##STR1##
This makes it inevitable to use a high-molecular weight compound as the silicon source.
Therefore, the partial hydrolysis process cannot be thought to be a process wherein an advantage of the liquid phase process that a homogeneous mixing of a multi-component system can be achieved through the use of a low-molecular weight starting material can be fully utilized.